1. Field of the Invention
The present invention relates to a laser oscillator that makes it possible to stably obtain a laser beam having a fundamental transverse mode pattern or a flat output intensity profile termed a top-hat pattern, in continuous and pulsed oscillation modes, even when there is a spatially non-uniform excitation distribution of the laser medium or the optical characteristics of the laser medium, or of the optical characteristics of the optical elements in the resonator of the laser oscillator.
2. Description of the Prior Art
Generally, a ring resonator or a Fabry-Perot resonator is used to form a stable optical resonator of a laser oscillator. The resonant mode of these resonators is a Hermite-Gaussian mode, denoted as TEMmn, where m and n signify a radial mode number and an azimuthal mode number for a round laser beam. The fundamental transverse mode is TEM00 mode, which is a Gaussian beam, considered the best oscillation mode because it possesses the smoothest intensity distribution and high convergence.
In TEM mode, the optical wavefront coincides with the surface shapes of the reflecting mirrors at each end of the resonator. This means that a light beam reflected at a point on the mirror will return back to this point after a round trip in the resonator ignoring the beam divergence caused by diffraction. In other words, the laser beam in the resonator is localized spatially within the beam cross-section. Also, the excitation distribution of the laser medium used in the laser oscillator, the refractive index distribution of the optical elements and other such spatial distributions, also birefringence and the like caused by thermal distortion in the laser rods, has a close correspondence to the amplitude-phase distribution of the laser light in the resonator.
This means that if the spatial characteristics of the laser medium or optical elements are non-uniform, the amplitude-phase distribution of the laser beam will also be non-uniform, distorting the beam wavefront and giving rise to corresponding higher-order transverse mode oscillations in laser oscillators. This being the case, much effort has been devoted to eliminating non-uniformity in the optical characteristics produced in the laser medium.
In the case of solid-state lasers, for example, the laser rods are inevitably required to be as uniform as possible optically. Also, to obtain fundamental transverse mode oscillation, the excitation distribution in the laser rod is required to be at least a flat or Gaussian function type. For this, it has been necessary to use a plurality of exciting light sources to realize uniform excitation. Moreover, when the solid-state laser rod is strongly optically excited, it produces a non-uniform refractive index distribution and thermal birefringence, which have to be compensated for by compensating optical system inside the resonator. For the solid-state lasers it has generally been necessary to provide a compensation optical system inside the laser resonator to deal with such non-uniformity.
In addition, Gaussian beam has the maximum intensity at the beam center, so the optical components of the resonator are easily damaged by this center part of the beam. This has been the biggest reason why the optical output of the laser cannot be increased in Gaussian mode. That is what has made it difficult to increase the output power of lasers in fundamental transverse mode.
Another obstacle to realize the uniformities is the fact that, in the case of a solid-state laser, the laser medium is excited by external light sources, so that if a simple exciting light source is used, the excitation density tends to be higher at the irradiated periphery of the laser rod than at the center. On the contrary, as described, the beam intensity is lowest at the periphery. As a result, energy stored by the excitation cannot be efficiently converted to laser light. This has also been another obstacle to boosting the energy conversion efficiency of solid-state laser devices.
In contrast to the above stable Fabry-Perot resonator, laser light obtained from a pulsed laser oscillator, which uses an unstable Fabry-Perot resonator, exhibits what is called a top-hat pattern of the intensity profile, which is substantially flat from the center to the periphery. This laser beam has the same good convergence like a Gaussian beam. In addition, beam energy can be increased without loss of lasing stability, giving it very good quality for applications such as machining and scientific research. As described in the article “Mode calculations in unstable resonators with saturable gain, 2: Fast Fourier transform method,” E. A. Sziklas, et al., Applied Optics, Vol. 14 (8), pp. 1874-1889 (1975) [Reference 1], top-hat mode has been thought to be based on TEM mode in the vicinity of the beam axis. However, according to the recent research described in “Fractal modes in unstable resonators,” G. P. Karman, et al., Nature, Vol. 402, p. 138 (Nov. 11, 1999) [Reference 2], and “Fractal structure of eigenmodes of unstable-cavity lasers,” G. P. Karman, et al., Optics Letters, Vol. 23 (24), pp. 1909-1911 (1998) [Reference 3], the intensity distribution exhibits a highly fractal property, and according to “Kaleidoscope laser,” G. S. McDonald, et al., Journal of the Optical Society of America B, Vol. 17, No. 4, pp. 524-529 (2000) [Reference 4], laser intensity distributions obtained by numerical calculation exhibited a fractal structure. From these articles it is convinced that a top-hat mode differs from the TEM modes. Performances of a laser emitting a beam output of top-hat pattern in continuous or pulsed oscillation modes using other optical resonators than the unstable FP resonator is expected to be improved in conversion efficiency and beam quality and stability and the like.
Previously, the stable existence of a top-hat mode in a resonator was impossible to make. Even if a light wave of flat intensity profile is generated at an instant in a stable Fabry-Perot resonator, diffraction of light will strongly modify the top-hat profile of the amplitude-phase distribution only in one round-trip of the resonator, making it impossible to sustain the initial intensity profile.
An object of the present invention is to provide a laser oscillator, in which a laser beam by continuous or pulsed oscillation exhibits a uniform intensity profile or the fundamental transverse mode and also the conversion efficiency from exciting light to the laser output energy is high.
In other words, an object of the present invention is to provide a laser oscillator that makes it possible to obtain a fundamental transverse mode or top-hat mode of a laser beam in continuous oscillation or in pulsed oscillation with high stability and efficiency.